In general, an output driver circuit of a semiconductor memory device is used to output an internal data to the outside of a chip through an output terminal, that is, a data pad. The output driver circuit receives the internal data and drives the data pad with a set driving strength.
FIG. 1 is a circuit diagram of an output data driving unit included in a conventional output driver circuit.
As illustrated in FIG. 1, the output data driving unit included in the conventional output driver circuit receives a pull-up driving signal PUB and a pull-down driving signal PD, which are generated by pre-decoding an internal data, and drives a data pad DQ. At this time, driving strength which drives the data pad DQ is determined by pull-up codes PCALB<1:4> and pull-down codes NCALB<1:4>. The pull-up codes PCALB<1:4> and the pull-down codes NCALB<1:4> are generated by a ZQ calibration circuit.
The ZQ calibration circuit is a circuit that counts the pull-up codes PCALB<1:4> and the pull-down codes NCALB<1:4> by using an external resistor ZQ having a constant resistance, regardless of variations in process, voltage and temperature (PVT) characteristics, in order to drive the data pad DQ with a constant driving strength even though the PVT characteristics are varied. That is, the ZQ calibration circuit up-counts the pull-up codes PCALB<1:4> and the pull-down codes NCALB<1:4> when it is necessary to increase the driving strength which drives the data pad DQ, depending on the variations in the PVT characteristics. On the other hand, the ZQ calibration circuit down-counts the pull-up codes PCALB<1:4> and the pull-down codes NCALB<1:4> when it is necessary to decrease the driving strength, depending on the variations in the PVT characteristics.
In the conventional output data driving unit, PMOS transistors P11, P12, P13, P14 receiving the pull-up driving signal PUB are coupled in series to PMOS transistors P15, P16, P17, P18 receiving the pull-up codes PCALB<1:4>, respectively. Also, NMOS transistors N11, N12, N13, N14 receiving the pull-down driving signal PD are coupled in series to NMOS transistors N15, N16, N17, N18 receiving the pull-down codes NCALB<1:4>, respectively. As such, in a case where the output data driving unit is configured with two or more MOS transistors coupled in series, the operating speed and the driving strength are greatly reduced, as compared to a case where the output data driving unit is configured with single MOS transistors. Thus, sizes of the MOS transistors must be increased in order to ensure the same operating speed and driving strength. Consequently, the conventional output data driving unit must use large-sized MOS transistors, resulting in the increase in the circuit area and current consumption.